Constant Power vs Constant Current

On: Thu, Apr 18, 02 12:42:30 AM

Colin Kaminski wrote:

I have had very experienced laser designers recommend both
constant current and constant output power. Tom, can you
provide me with some guidance? I have been using Frank's
circuit with constant power (APC). Should I try constant
current? What driver would be versatile? I am looking to
start trying different diodes.

RESPONSES

Tom B. - Thu, Apr 18, 02 04:45:21 AM

I'm in the constant current camp myself. The weak link
in the APC scheme is the photodiode, which is quite
sensitive to back-reflections and other nastiness. One
perceived disadvantage of constant current drive is the
need to hold the diode temperature constant, either by
actively controlling it or running in a constant
temperature environment (like my basement). But you
have to hold the temperature constant anyway to avoid
mode-hopping. My reasoning is that if you have to
hold the temperature constant anyway, and at a given
temperature output power is constant for a given drive
current, why go through the rigamorole of amplifying a
weak constant signal (and added noise) from the photodiode
to produce what should be a constant current, but won't
be because of extra noise and back-reflections. Worse
yet, the thing most likely to change the amount of light
relected back to the laser is opening the shutter, and
you don't want the APC circuit to start responding to an
apparent change in output power just as your exposure starts.
The simplest constant current source is a stable
battery and a variable resistor. Or a well filtered
variable lab supply and a fixed resistor. I'm using
both, and they seem to work well enough. But I would really
rather have a low noise variable current source that's
a little more high-tech. Eventually I'll get around to
designing or buying one. In the affordable category,
I like the Thorlabs LD1255 0-250 mA driver - the 1 microAmp
RMS noise spec looks very good. With the cable and
power supply, it runs to a total of $140 US.
(www.thorlabs.com) I seem to recall looking at some drivers
and chips from other companies and finding that they didn't
bother to provide noise specs - not a good sign.
24.67.253.203

Colin - Mon, Jun 03, 02 01:27:13 PM

OK, you talked me into it. I just ordered the LD1255. I'll
let you know how it works.
64.170.195.49

Tom B. - Fri, Jun 07, 02 02:52:12 AM

Cool - I'd be interested in your impressions as I've
only seen the catalog description. For general purpose
testing, I'd be tempted to put the thing in a ferrous
metal box to shield it from stray magnetic fields and
replace the little trimmers with good quality front-panel
pots (multiturn). Also add a digital panel meter module
to monitor drive current. And one for temperature, too,
of course. And so on. Thus do simple projects grow into
multi-weekend monsters.24.67.253.203

Colin - Fri, Jun 07, 02 11:22:55 PM

You would save about $120 to just start from scratch. Low
noice circuitry is not that much harder than a simple
current source. 64.170.193.244